Conventionally, as processes for producing a sulfur-containing hydroxycarboxylic acid, a process for hydrolyzing cyanhydrin using sulfuric acid as a catalyst, a process for hydrolyzing a hydroxynitrile compound through action of a microorganism to convert into a corresponding hydroxycarboxylic acid (e.g., Japanese Patent Laid-Open for Opposition No. 58-15120, Japanese Patent Application Laid-Open (JP-A) No. 2-84198, JP-A-4-40898), and the like are known.
However, in the process using sulfuric acid as a catalyst, the reaction between a hydroxynitrile compound and sulfuric acid produces the objective compound, a hydroxycarboxylic acid, and an equimolar amount of ammonium sulfate as a byproduct. Accordingly, since the process requires a step for collecting the byproduct, there is a problem that the steps are made complicate and production costs increase.
Furthermore, the process for producing a hydroxycarboxylic acid compound from a corresponding hydroxynitrile compound using a microorganism has problems that cyanogen and the like generated by degradation of the hydroxynitrile compound inhibit the enzymatic activity of the microorganism; and that the production costs increase since the process requires desalting treatment of the produced ammonium salt, and the like.
The present inventors have investigated to find a process for producing a sulfur-containing hydroxycarboxylic acid that does not use a hydroxynitrile compound as a raw material, and found that the primary hydroxyl group of a sulfur-containing dihydroxy compound can be preferentially oxidized by using a cell or a material from treated cell of a microorganism that can convert a sulfur-containing dihydroxy compound to a corresponding α-hydroxycarboxylic acid compound, resulting in the completion of the present invention.